I would have probably allowed the bagel because its not really junk food. But after that I would have put him in his room and then just asked him what was making him so frusterated. Maybe offered another snack?

I would have handled it the exact same way. My 3 year old is notorious for throwing fits when told he cant have something though. He hasnt tried to hit or kick me in a long time though. Arent bagels healthy though?